The present invention relates to a semiconductor pressure converting device and in particular to a semiconductor converting device capable of obtaining a static pressure signal together with a pressure differential signal.
An example of a composite function type semiconductor pressure converting device capable of obtaining a pressure differential signal and a static pressure signal is disclosed in JP-A-No. 58-120142. In the device disclosed in the relevant publication, one set of semiconductor resistors sensitive to pressure differences are formed in portion of a silicon diaphragm having a smaller thickness and one set of semiconductor resistors sensitive to static pressure are formed in a portion having a greater thickness and constituting the outer periphery of the diaphragm. The silicon diaphragm is secured by adhesion to a fixing table made of glass.
Now, at a measurement of the pressure difference, since static pressure is applied uniformly to the semiconductor pressure converting device, extremely great compressive stress is produced in the set of resistors for detecting the pressure difference formed in the portion of the silicon diaphragm having a small thickness. Further, since the static pressure is applied also to the fixing table, bending stress is produced in the silicon diaphragm due to the difference in the deformation between the silicon diaphragm and the fixing table, which gives rise to errors in the setting of the zero point for the static pressure, compared with the zero point in the case of the atmospheric pressure. (In general these errors are called static pressure influence.) Since these errors are superposed on the pressure difference signal at the pressure difference measurement, the pressure difference signal includes errors. For this reason the static pressure is detected by forming a set of semiconductor resistors in the great thickness portion with the intention of correcting errors due to this static pressure.
The output level of this static pressure signal is one of several parts tens with respect to that of the pressure difference signal and therefore it is very small. The principle of the output of the static pressure signal utilizes the strain difference between the strain produced in the fixing table and that produced in the silicon diaphragm at the application of the static pressure, based on the difference in material between the fixing table and the silicon diaphragm. Consequently, in order to obtain a static pressure signal of high output level, it is necessary to have a strain difference as great as possible. However, if the strain difference is great, a great strain difference takes place also in the junction portion between the silicon diaphragm and the fixing table. Thus the strength of the junction portion comes into question and the tolerable applicable static pressure, i.e. the static pressure strength is lowered. Further, since an increase in the strain difference naturally produces great influences on the small thickness portion of the silicon diaphragm, interference between the pressure difference signal and the static pressure signal becomes large and it is difficult to obtain a pressure difference signal with a high precision, corrected with the static pressure signal of high output level.
That is, there was a problem that if the static pressure signal is increased, the static pressure strength is lowered and the interference between the static pressure signal and the pressure difference signal becomes large. This makes it difficult to obtain a corrected pressure difference signal with a high precision.